Technical Field
This disclosure relates to electronic circuits and, more particularly, to current sources which produce a transient output current that is insensitive to temperature variations.
Discussion of the Related Art
A current mirror is a type of current source that copies an input current to an output current. The input and output currents can be the same or different, depending on the components of the current minor circuit. The current mirror can provide bias currents or can serve as an active load. A basic current mirror includes two transistors having their gate terminals connected together. As a variation, a cascode current minor includes a cascode transistor connected in series with each of the current mirror transistors. The steady state output current of a cascode current mirror is relatively insensitive to temperature variations.
In some applications, the output current of the cascode current minor is switched on and off. For example, the current minor may be used to discharge a capacitor for a determined discharge period. In such applications, the output of the current mirror is connected through a switch to the capacitor to be discharged. The switch is closed for the discharge period, and the constant current of the current mirror causes the capacitor voltage to decrease linearly. An example of an application is the discharge of the capacitance of a touch screen display in a mobile device.
In certain applications, including but not limited to mobile devices, stable operation of the current source over a range of temperatures is desirable. As noted above, current minors are relatively insensitive to temperature variation in steady state operation. However, when the output current is switched on and off, the operation of the circuit may be sensitive to temperature variations. Accordingly, there is a need for current sources which are relatively insensitive to temperature variations under transient operating conditions.